The study is a multi-center randomized trial to determine if tamoxifen in a daily dose of 20 mg reduces the risk of developing breast cancer compared to placebo. Women eligible for the study include women over the age of 60 or women 35 to 59 who have an estimated risk of developing breast cancer similar to that of a 60 year old woman. Over 50 women have been screened and 5 have been randomized. The majority of women refuse participation because of either potential side effects of tamoxifen or the wish to continue estrogen replacement therapy. One woman is now off the study after developing uterine cancer. Another woman went off the study because of side effects of the study pills. The duration of the trial is for at least 10 years. In the process of recruiting for the trial, we have been able to identify women appropriate for participation in a case-control molecular epidemiologic study of susceptibility factors to breast cancer.